This invention relates to motor drive and control circuits for electric motors, in particular for use in an electric power steering system.
Electric power steering systems are known where an electric motor applies a torque to assist the driver in turning the steering wheel, the amount of torque being dependent on the amount of torque applied to the steering wheel by the driver. A torque sensor is provided that measures the torque in the steering, typically in the steering shaft, and a microprocessor determines a torque demand signal for the motor dependent upon the measured torque as set by a defined boost curve. This demand signal is supplied to a motor controller which generates a motor drive signal that is passed to a motor drive circuit. The motor drive circuit modulates the switches of a motor drive stage which in turn controls the current flowing through the phases of the motor. The actual current flowing in the motor may be measured using a current sensor, or estimated in a sensorless control system, and this may be fed back into the controller to provide a closed loop control of the motor.
To provide resilience the motor may be provided with two sets of windings, each independent of the other, and each being driven by a respective drive circuit. In normal use only one drive circuit and set of windings may be used to drive the motor, but when a fault is detected the circuit can switch over to use the other set of windings and drive circuit. Alternatively, both sets of windings can be driven in normal use and when a fault occurs with one then the remaining set of windings can continue to drive the motor. This is known as a dual lane system. In one arrangement, each drive circuit can be controlled by a respective separate control circuit, and each control circuit can be supplied with power from its own separate power supply.
A similar resilience can be achieved by using two mechanically linked motors, each one being provided with one of the two lanes of the dual system.
Providing two lanes allows the motor (or one of a pair) to continue to provide assistance after a fault. In order to achieve greater availability after the first fault, high levels of flexibility between the lanes needs to be achieved so that failures of significant portions of the circuit can be mitigated without reducing the capability of the design. This will require increased numbers of signals to be sent between the lanes to permit each lane to take over more of the functionality of the failed lane. A lane needs to be able to monitor the signals from the other lane in order to identify that it needs to take over.